Many industrial applications of water require the use of ultrapure water, which is water that has a very low organic carbon content, i.e. less than about twenty-five parts per billion. For example, oxidation of substrates during manufacture of integrated electronic circuits can be deleteriously affected by the presence of organic deposits on the substrate surface. Ultrapure water is often employed to dissolve organic deposits from substrate surfaces prior to oxidation. Other typical applications for which ultrapure water is employed include the manufacture of medical devices, chemicals, and analytical laboratory applications.
Ultrapure water is typically prepared by methods which include, for example, filtration, ion exchange and exposure to ultraviolet radiation. However, the ability of a water purification system to reduce the organic carbon content of water to acceptable levels varies according to the conditions of filtration, such as the temperature and flow rate of the water through the purification system. Also, the effectiveness of purification is generally dependent, at least in part, upon the quantity and type of organic contaminants within the water being purified. Consequently, the organic carbon content of ultrapure water can vary significantly during minor changes in the operating conditions of purification.
One attempt to monitor the organic carbon content of ultrapure water is based on oxidation of organic carbon to form carbon dioxide, such as by exposure of the water to ultraviolet light. For example, Patent Cooperation Treaty (PCT) patent application PCT/JP90/01434, by Nihon Millipore Kogyo Kabushiki Kaisha, published May 16, 1991 as WO91/06848, discloses a method and apparatus for monitoring the organic carbon content of ultrapure water by monitoring the difference in electrical resistivity of the water across an ultraviolet light during purification. The value of the organic carbon content of water discharged from the water purification system is presumed, based on a known organic carbon content of the water prior to exposure to ultraviolet light and on an observed drop in resistivity across the light. However, other factors may affect the carbon content and thus can cause an error in the presumed value. Examples of such factors include changes in the temperature and flow rate of the water in the purification system, the nature of the organic carbon content in the water, and aging of components in the purification system. Another limitation of this prior art system involves the fact that oxidation products emanating from the ultraviolet light are not always the same. It is known that a particular resistivity change could be produced by different levels of different oxidation products. For example, if carboxylic acids are formed as an oxidation product, the resulting resistivity change could be the same as that resistivity change produced by a different level of carbon dioxide, yet the measured total organic carbon content of these two oxidation products could be different. Thus, this system does not take into account variances of the oxidation products produced. Due to the diversity of organic compounds found in feed waters, this is an important factor to consider in the design of a water purification system.
Also, known apparatus for estimating the total organic carbon content can be cumbersome when used in conjunction with water purification systems. An example of an apparatus for estimating the total organic carbon content of ultrapure water is disclosed in U.S. Pat. No. 5,047,212, issued to Blades et al. (hereinafter "Blades et al."). Blades et al. disclose exposure of a sample of ultrapure water to ultraviolet light and recording of the electrical resistivity of the water over time during exposure to the ultraviolet light. However, the system and method disclosed by Blades et al., is time consuming. For example, a single estimate of the organic carbon content of a sample of ultrapure water withdrawn from a water purification system can require about one-half hour.
Further, water purification systems are typically operated on a continuous basis. Also, the conditions of the water purification system and of the water directed into the system can vary significantly during analysis of the sample withdrawn from the system. Consequently, the estimated organic carbon content value obtained by Blades et al., can be obsolete before the value can be reported. As a result, monitoring of the water purification system can be employed only when steady-state conditions of the system can be assured over extended periods of time.
Therefore, a need exists for a method and apparatus for predicting and monitoring the organic carbon content of water discharged from a water purification system which overcome or minimize the above-referenced problems.